Hitherto, as an internal sizing agent, a so-called rosin type sizing agent which employs rosin as raw material has been generally used. However, rosin is a natural substance and has some problems such as fluctuation of price, instability of supply and the like. Therefore, as synthesized sizing agents which take the place of this, so-called petroleum resin sizing agents which employ, as raw material, petroleum resin obtained from a mixture of C.sub.5 and C.sub.9 fractions which can be obtained from naphtha cracked oil and other various synthesized sizing agents have been proposed.
However, until several years ago, such synthesized sizing agents, except the petroleum resin sizing agent, were hardly put to practical use due to problems related to efficiency or price, and the petroleum resin sizing agent practically used was also limited in use due to problems related to efficiency.
However, recently substantial use has been made of synthesized sizing agents superior in efficiency and price to the rosin type sizing agent. Such agents are the water-soluble salt of alkenyl succinic acid which is made from a highly branched olefin and maleic anhydride. The paper sizing process using the water-soluble salt of alkenyl succinic acid as internal sizing agent has been well-known as described in Japanese Patent Publication No. 565/65 (corresponding to U.S. Pat. No. 3,139,373).
The water-soluble salt of alkenyl succinic acid has been said to have a problem in practical use due to foaming as described in Japanese Patent Publication (unexamined) No. 27003/79. The present inventors have found that the foam is formed only when the sizing agent is obstructed from fixing on pulp during sizing and the unfixed water-soluble salt of alkenyl succinic acid or a reaction product of alkenyl succinic acid with aluminum sulfate remains in a pulp slurry. Thus, the present inventors found that when fixing proceeds normally, there is no problem in practical use since there is almost no foaming. The undesirable obstruction to fixing of the sizing agent on pulp is herein referred to as "fixing obstruction".
However, though the obstruction to fixing of the sizing agent in the paper making process is a rare phenomenon, the foaming phenomenon caused by the unfixed sizing agent consisting of the water-soluble salt of alkenyl succinic acid gives a serious damage for its process and the quality of the produced paper.